Dose escalation will be conducted to determine the maximum tolerated dose (MTD). The dose levels of OMP 21M18 will be 0.5, 1.0, 2.5, 5, and 10 mg/kg administered IV weekly for 9 doses. No dose escalation or reduction will be allowed within a dose cohort. The dose may be administered at any time during the day. Three subjects will be treated at each dose level if no dose-limiting toxicities (DLTs) are observed. If 1 of 3 subjects experience a DLT, that dose level will be expanded to 6 subjects. If 2 or more subjects experience a DLT, no further subjects will be dosed at that level and 3 additional subjects will be added to the preceding dose cohort unless 6 subjects have already been treated at that dose level. Subjects will be assessed for DLTs from the time of the first dose through 7 days after administration of the 4th dose, but prior to administration of the 5th dose (i.e., Days 0-28). Dose escalation, if appropriate, will occur after all subjects in a cohort have completed their Day 28 DLT assessment.